Separation of liquid mixtures by distillation is a common chemical engineering operation. Distillation is most effective when the components of the liquid mixture have relative volatilities greater than about 1.5 to 2.0 in the entire compositional range of the mixture. However, when the components of the liquid mixture are close boiling, having relative volatilities of less than 1.5, separation by distillation becomes difficult, requiring a large energy expenditure and use of distillation columns having a large number of distillation trays.
Conventional distillation processes cannot be utilized if a liquid mixture forms an azeotrope or contains temperature sensitive components. Special distillation techniques such as azeotropic or extractive distillation, use of very high reflux ratios and vacuum distillation are typically used to separate these liquid mixtures. In some cases, fractional crystallization is required to effect separation. However, such operations are highly capital and energy intensive.
The limitations of conventional distillation techniques are particularly apparent when the liquid mixture forms an azeotrope (relative volatility being 1.0) at some relative composition of the mixture. Moreover, distillation cannot be used to separate components of a liquid mixture when the individual components cannot be heated above a certain temperature because of thermal degradation or reaction between the components. While vacuum distillation can be utilized, such a process significantly increases the energy expenditure and requires a large number of distillation trays to effect separation. Consequently, a need exists for an energy efficient process for separating liquid mixtures which contain close boiling components, while form azeotropes, which comprise a temperature sensitive component or the like.
U.S. Pat. No. 3,636,121 discloses a process for the selective adsorption of certain C.sub.8 aromatic isomers from a liquid mixture to produce individually concentrated streams of the individual C.sub.8 aromatic isomers while isomerizing a portion of the C.sub.8 aromatic stream to effect the additional production of a given C.sub.8 aromatic isomer. A first adsorption zone separates para-xylene and ethylbenzene from the other C.sub.8 aromatic isomers fed to that adsorption zone and passes the para-xylene and ethylbenzene to a second adsorption zone wherein para-xylene and ethylbenzene are separated into relatively purified para-xylene and ethylbenzene streams. The remaining C.sub.8 aromatics separated from the para-xylene and ethylbenzene in the first adsorption zone are passed into an isomerization zone to effect the production of additional para-xylene which is eventually recycled to the first adsorption zone allowing increased yield of para-xylene based on the C.sub.8 aromatics fed to the first adsorption zone. Ortho-xylene is recovered from the effluent of the isomerization reaction zone.
U.S. Pat. No. 4,024,331 discloses a process for separating a ketose from a liquid feed mixture containing a ketose and an aldose comprising contacting the mixture with an adsorbent comprising an X-zeolite containing one or more selected cations at the exchangeable cationic sites thereby selectively adsorbing a ketose from the feed mixture and thereafter recovering the ketose. Preferably, the ketose is recovered by desorption from the adsorbent with a desorbent material.
U.S. Pat. No. 4,306,107 discloses a process for separating a first component comprising meta-xylene, a second component comprising a mixture of para-xylene and ortho-xylene and a third component comprising ethyl benzene from a liquid feedstream employing an adsorbent comprising a Y-zeolite containing sodium ions at exchangeable cationic sites and a desorbent material comprising toluene which, in combination, have selectivities for the first, second and third components, respectively, in descending order of magnitude. The process uses a simulated-moving bed counter-current flow system with an intermediate raffinate stream taken off the column at about the midpoint of the adsorption zone in addition to the usual extract and raffinate product streams.